Successfully changing your user agent depends upon the web site you are trying to visit.

As this link, which you should read, clearly states: "We produced this page not because we think changing browser User Agent strings is a good idea - in fact we think it is an incredibly bad idea unless you absolutely need to do it."
http://www.zytrax.com/tech/web/change-ua.html

But this link will also explain the how's and why's of changing your user agent.